Abscisic acid (ABA) is a naturally occurring substance which is known to provoke retardation or inhibition of growth in plants. The response of a plant to abscisic acid may be brief or prolonged. As mentioned at p. 526 of Abscisic acid, Addicott, ed., it may either be a simple, temporary suspension of growth, or it may modify the morphological pattern of growth.
Abscisic acid is a hormone found in all higher plants. Because of the interesting growth properties that were attributed to abscisic acid, extensive research was conducted to find suitable derivatives that could possibly possess the same or superior activities when compared to the natural plant growth regulator.
The ability of abscisic acid to arrest growth of woody shoots and to contribute to the dormancy of apical buds was detected for the first time by Eagles and Wareing in 1963, Nature 199, 874-876. Furthermore, Milborrow demonstrated in 1974 (1974, Rev. Plant Physiol. 25;259-307) that abscisic acid had an inhibiting effect on the germination of intact seeds and isolated embryos. In fact, one standard bioassay for abscisic acid utilizes the germination of wheat embryos, which is affected by concentration of suitable strength.
Six-membered carbocyclic substituted 1,3-butadiene compounds, such as 1-hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexene-1-penta-2,4-dienoic acid methyl ester have been prepared and described in U.S. Pat. No. 3,576,839. These compounds have been used in pre-emergent herbicidal applications to delay the germination of seeds and in post-emergent applications to defoliate and to effect leaf senescence.
In 1969, Saburo Tamura and Minoru Nagao prepared analogs of abscisic acid and demonstrated that these compounds were useful as growth inhibitors. It was demonstrated that the most active derivatives prepared by Tamura and Nagao possessed a growth inhibitory activity that was comparable or superior to that of abscisic acid. It was also discovered that these compounds significantly counteracted the action of gibberellin A.sub.3. In 1981, two other Japanese scientists, Takayuki Oritani and Kyohei Yamashita published another series of results concerning derivatives of abscisic acid, mentioning that these compounds possessed strong growth inhibitory activities on plants that were comparable to those of abscisic acid (Agric. Biol. Chem 46 (3), 817-818 (1982)). The compounds synthesized by these two groups of scientists all had in common the carbon skeleton of abscisic acid.
In 1986, U.S. Pat. No. 4,581,057 issued to Nooden demonstrated a further series of abscisic acid derivatives that could this time be used to enhance the absorption of nutrients by plants. The discovery was important in that it provided a tool that could be used for enhancing the rate of absorption of fertilizers into the plants. It therefore had the implication of reducing the amount of fertilizers that had to be used. The derivatives prepared by Nooden in U.S.Pat. No. 4,581,057 could be used "to obtain the desired enhancement in translocation of nutrients to the reproductive tissues and other plant parts".
Therefore, since abscisic acid was first isolated by Ohkuma et al. in 1963, a large number of derivatives of that compound have been prepared and have been used for inhibiting the growth of plants.
The use of germination and growth promoters in agriculture, forestry, horticulture and malting is a widely developed practice. Growth promoters are products that shorten the time necessary for a crop to mature and thus permit greater security of harvest in short-season climates such as are found in Canada and Northern Europe. However, various problems are associated with the use of these products. In most instances, these products are expensive and must be used in very large amounts thereby causing important environmental problems. Also, problems with secondary growth and yield decrease have been noted.
Therefore, it would be highly desirable to produce and develop germination and growth promoting agents that could be used efficiently in small concentrations.
The freezing tolerance of tissue cultures can be enhanced by treating cultures with abscisic acid. For example, bromegrass cell suspension cultures treated with 75 .mu.M ABA for 7 days can withstand freezing to -40 .degree. C. (Reaney and Gusta 1987, Plant Physiol., 83:423). Results on whole plants are conflicting in that ABA can increase, decrease or have no effect on freezing tolerance. No practical application of ABA or ABA analogs for enhancing freezing tolerance of plants has been reported.
ABA at concentrations as low as 10.sup.-8 M acts as an antitranspirant in partially closing stomata (N. Kondo, I. Maruta and K. Sugahara, 1980, Plant. Cell. Physiol., 21:817). Stomata may remain partially closed for as long as 4 days after treatment with 10.sup.-4 M ABA and an acetylenic ABA aldehyde analog (H. Schaudolf, 1987, J. Plant Physiol., 131:433). This analog decreases water use in Helianthus annuus, Triticum aestivum and Lycopersicon esculentum while maintaining yield.
Over 80 percent of transplants (Capsicum annuum plants dipped into a solution of ABA prior to planting in dry soil survived while less than 60 percent of control plants survived (G. A. Berkowitz and J. Rabin, 1988, Plant Physiol. 86:344). Furthermore, the treated plants had a 30 percent higher yield.
However, the use of ABa in freezing resistance and antritranspiration experiments on plants presents serious drawbacks. Firstly, abscisic acid is very expensive to product commercially and secondly, the desired effect is only observed for short periods of time because ABA, a naturally occurring hormone, is rapidly degraded by microorganisms found on the plants or by the plants themselves.